This invention relates to improvements to semiconductor structures and more particularly to quantum well structures that are made to undergo interdiffusion disordering after crystal growth.
Interest in the formation of different types of heterostructures in the AlAs-Ga.sub.1-x Al.sub.x As-GaAs alloy system have been further advanced by interdiffusion techniques and, in particular, interdiffusion by disordering of GaAs and AlAs or disordering Ga and Al in the Ga.sub.1-x Al.sub.x As alloy. The use of interdiffusion disordering is of high interest because the selective treatment of these alloy systems after crystal growth promises the possible fabrication of semiconductor lasers, optical waveguides, optical detectors, and III-V integrated electro-optical circuits. Interdiffusion disordering techniques will provide the selective control of regions in as-grown quantum well structures to be "scrambled" or disordered in alloy component content and thereby shifted up in energy gap and reduction in refractive index to form active or waveguide regions in an ordered-crystal structure previously grown using conventional epitaxial methods.
Examples of interdiffusion disordering between Ga and Al in Ga.sub.1-x Al.sub.x As alloy systems or GaAs-AlAs layers or superlattices are disclosed in the following three sets of publications:
(1) L. L. Chang et al., "Interdiffusion between GaAs and AlAs", Applied Physics Letters, Vol. 29(3), pp. 138-141 (Aug. 1, 1976) involves interdiffusion disordering via annealing.
(2) U.S. Pat. No. 4,378,255; W. D. Laidig et al., "Disorder of an AlAs-GaAs Superlattice by Impurity Diffusion", Applied Physics Letters, Vol. 38(10), pp. 776-778 (May 15, 1981); and S. W. Kirchoefer et al., "Zn Diffusion & Disordering of An AlAs-GaAs Superlattice Along its Layers", Journal of Applied Physics, Vol. 53(1), pp. 766-768 (January 1982) all involve interdiffusion disordering via zinc diffusion at temperatures, for example, in the range of 600.degree. C.-650.degree. C.
(3) M. D. Camras et al. "Disorder of AlAs/GaAs Superlattices by the Implantation and Diffusion of Impurities", Proceedings of the International Symposium on GaAs and Related Compounds in Albuquerque, N. Mex. in 1982, pp. 233-239, involves interdiffusion disordering via ion implantation, e.g., Si or Zn ion implantation at a temperature, for example, at about 675.degree. C.
Recently, interdiffusion disordering techniques have been extended to single and multiple quantum well laser structures for wavelength modification of their output by interdiffusing Ga and Al at the active layer heterojunctions at high temperatures, for example, in the range of 850.degree. C.-1000.degree. C. By controlling the temperature and time of annealing, the wavelength of the laser structure may be shifted within a range as much as 1000 .ANG., for example, within the range from 8200 .ANG. to 7200 .ANG.. This technique is disclosed in U.S. patent application Ser. No. 528,766 filed Sept. 2, 1983 entitled "Wavelength Tuning of Quantum Well Lasers By Thermal Annealing" and assigned to the assignee herein now U.S. Pat. No. 4,585,491.
In the case of interdiffusion disordering, via elemental implementation, elemental diffusion or thermal annealing at high temperatures, of a single or multiple quantum well structures, e.g., heterostructure quantum well lasers, the energy band profile of the well will be modified according to the extent of applied interdiffusion disordering treatment. In the case of AlAs-GaAs interfaces, GaAs-GaAlAs interfaces or Ga.sub.1-x Al.sub.x As-Ga.sub.1-y Al.sub.y As (y&gt;x) interfaces, the Al composition gradient across the structure will not be as abrupt as in the case of the epitaxially as-grown structures prior to interdiffusion disordering treatment.
FIGS. 1A-1C illustrate various quantum well structures in the GaAs/GaAlAs regime known in the art. In FIG. 1A, the quantum well structure for the energy band profile 10 comprises a well layer 12 of Ga.sub.1-x Al.sub.x As where x may be in the range of approximately 0 to 0.35, i.e., layer 12 may be a GaAs layer or a low Al content layer of GaAlAs. In order to exhibit a quantum size effect, layer 12 may have a thickness in the range of 15 .ANG. to 500 .ANG.. The cladding layers 15 and 16 comprise Ga.sub.1-x' Al.sub.x' As where x' may be in the range of approximately 0.15 to 1.00, i.e., layers 15 and 16 may be high Al content layers of GaAlAs or AlAs.
Interdiffusion disordering will cause an interdiffusion between Ga and Al at the interfaces between the quantum well region 12 and the cladding layers 15 and 16. As illustrated in FIG. 1A, the original energy band profile 10 forming the single quantum well region 12 has a finite square-shaped well. With interdiffusion disordering, the initially finite square-shaped well becomes more parabolic-like in shape, converting the well into a shallower, rounded edge Ga.sub.1-x Al.sub.x As well due to Ga-Al interdiffusion. This altered profile is illustrated by dotted line 13. This "shallower" of the well 12 shifts the confined particle electron and hole states of the well to different energy levels. A more detailed discussion concerning these states may be found in U.S. Pat. No. 4,585,491 supra.
The energy band profile 20 in FIG. 1B comprises a similar quantum well structure but includes the further outer layers 17 and 18 of Ga.sub.1-x" Al.sub.x" As where x" may be in the range of approximately 0.30 to 1.00, i.e., layers 17 and 18 may be high Al content layers of GaAlAs or Al while intermediate cladding layers 15 and 16 may be in the range of approximately 0.15 to 0.85, i.e., layers 15 and 16 may be intermediate Al content layers of GaAlAs. With sufficient interdiffusion disordering, the initially finite square shaped well 12 becomes more curved or parabolic-like, as illustrated by the altered profile shown by dotted line 14. To be noted is that the interdiffusion disordering in profile 20 has been more substantial as compared to profile 10, as more disordering is shown to have taken place "filling" the well 12.
The energy band profile 30 of FIG. 1C represents a superlattice of the single well of FIG. 1B and comprises four quantum wells 12A, 12B, 12C and 12D of Ga.sub.1-x Al.sub.x As where x is the range of approximately 0 to 0.35. The wells are separated by three barrier layers 15A, 15B and 15C comprising Ga.sub.1-x' Al.sub.x' As where x' is in the range of 0.15 to 0.85. It is to be noted that layers 15A, 15B and 15C could alternatively be AlAs. With interdiffusion disordering, the initially square shaped wells 12A, 12B, 12C and 12D become curved line shaped as depicted by the dotted ine 19 in FIG. 1C.
There are situations where it is necessary to ensure that in the epitaxial growth of interfaces to form quantum well structures, the well width at the top of wells is required to be controlled or is desired not to be too wide. Also, longer processing times in the application of interdiffusion disordering techniques are not desirable from the standpoint of device yield.